Typical electrostatographic printing machines (such as photocopiers, laser printers, facsimile machines, or the like) employ an imaging member that is exposed to an image to be printed. Exposure of the imaging member to the image to be printed or to a scanned image containing beam records an electrostatic latent image on the imaging member corresponding to the informational areas contained within the image to be printed. Generally, the electrostatic latent image is developed by bringing a toner or developer mixture into contact therewith.
One type of developer used in such printing machines is a liquid developer comprising a liquid carrier having toner particles disposed therein. Generally, a suitable colorant such as a dye or pigment, a charge director and a suitable binder are present in the toner particles. The liquid developer is brought into contact with the electrostatic latent image and the colored toner particles are deposited thereon in image configuration.
Developed toner images recorded on the imaging member may be transferred to an image receiving substrate such as paper or clear plastic via an intermediate transfer member. Transfer of the toner particles from the imaging member to the intermediate transfer member is conventionally accomplished electrostatically by means of an electrical potential between the imaging member and the intermediate transfer member. After the toner image has been transferred to the intermediate transfer member, it is then transferred in image configuration to the image receiving substrate, such as by contacting the substrate with the image on the intermediate transfer member under heat and/or pressure.
The use of an intermediate transfer member enables high throughput at modest process speeds. In color systems, the intermediate transfer member also improves registration of the final color toner image. Intermediate transfer members also allow for transfer of toner images to a broader range of substrates, including paper, plastics, etc. A disadvantage of using an intermediate transfer member is that a plurality of transfer steps is required. In the process of electrostatically transferring toner images from the imaging member to an intermediate transfer member, charge exchange can occur between toner particles and the transfer member, leading to less than complete toner transfer and poor image quality.
Intermediate transfer members employed in imaging apparatuses should exhibit substantially 100% transfer of toner particles from the imaging member to the intermediate transfer member and substantially 100% transfer of toner particles from the intermediate transfer member to the image receiving substrate. Substantially 100% toner transfer occurs when most or all of the toner material comprising the image is transferred and little or no residual toner remains on the surface from which the image was transferred. Substantially 100% toner transfer is particularly important when the imaging process involves generating full color images, since undesirable color shifting or degradation in the final colors obtained can occur when the primary color images are not efficiently transferred from the intermediate transfer member to the image receiving substrate.
Imaging processes wherein a developed image is first transferred to an intermediate transfer member and subsequently transferred from the intermediate transfer member to an image receiving substrate are known.
U.S. Pat. No. 4,796,048 (Bean) discloses an apparatus which transfers a plurality of toner images from a photoconductive member to a copy sheet. A single photoconductive member is used. The apparatus may include an intermediate transfer belt to transfer a toner image to a copy sheet with the use of a biased transfer roller. The intermediate transfer belt has a smooth surface, is non-absorbent and has a low surface energy.
U.S. Pat. No. 4,708,460 (Langdon) discloses an intermediate transport belt that is preferably made from a somewhat electrically conductive silicone material having an electrical conductivity of 10.sup.9 ohm-cm so that the belt is semiconductive. The apparatus includes a single photoconductive drum.
U.S. Pat. No. 4,430,412 (Miwa et al.) discloses an intermediate transfer member, which may be a belt-type member that is pressed onto an outer periphery of a toner image retainer with a pressure roller. The intermediate transfer member is formed with a laminate of a transfer layer comprising a heat resistant elastic body such as silicone rubber or fluororubber, and a heat resistant base material such as stainless steel. Silicone rubber is the only material shown in the examples as the transfer layer. A single layer fluorocarbon elastomer is not disclosed or suggested for the transfer member.
U.S. Pat. No. 3,893,761 (Buchan et al.) discloses a xerographic heat and pressure transfer and fusing apparatus having an intermediate transfer member which has a smooth surface, a surface free energy below 40 dynes per centimeter and a hardness from 3 to 70 durometer (Shore A). The transfer member, preferably in the form of a belt, can be formed, for example, from a polyamide film substrate coated with 0.1-10 millimeters of silicone rubber or fluoroelastomer. Silicone rubber is the only material shown in the example as the transfer layer. A single layer fluorocarbon elastomer is not disclosed or suggested.
U.S. Pat. No. 5,099,286 (Nishishe et al.) discloses an intermediate transfer belt comprising electrically conductive urethane rubber reportedly having a volume resistivity of 10.sup.3 to 10.sup.4 ohm-cm and a dielectric layer of polytetrafluoroethylene reportedly having a volume resistivity equal to or greater than 10.sup.14 ohm-cm.
U.S. Pat. No. 5,208,638 (Bujese et al.) relates to an intermediate transfer member comprising a fluoropolymer with a conductive material dispersed therein as a surface layer upon a metal layer, which in turn is upon a dielectric layer. The use of fluorocarbon elastomers is disclosed along with numerous other fluoropolymer materials, but there is no disclosure or suggestion of the improved efficiency achievable by the use of fluorocarbon elastomer, particularly without a substrate.
U.S. Pat. No. 5,233,396 (Simms et al.) discloses an apparatus having a single imaging member and an intermediate transfer member which is semiconductive and comprises a thermally and electrically conductive substrate coated with a semiconductive, low surface energy elastomeric outer layer that is preferably Viton.RTM. B-50 (a fluorocarbon elastomer comprising a copolymer of vinylidene fluoride and hexafluoropropylene).
U.S. Pat. Nos. 4,684,238 (Till et al.) and 4,690,539 (Radulski et al.) disclose intermediate transfer belts composed of polyethylene terephthalate or other suitable propylene materials. A single photoconductive drum is disclosed.
U.S. Pat. No. 5,119,140 (Berkes et al.) discloses a single layer intermediate transfer belt preferably fabricated from clear, carbon loaded or pigmented Tedlar.RTM. (a polyvinylfluoride available from E.I. du Pont de Nemours & Co.). Tedlar.RTM. suffers from poor conformability. The apparatus utilizes four individual image forming devices.
U.S. Pat. No. 5,298,956 (Mammino et al.) discloses a seamless intermediate transfer member comprising a reinforcing belt member coated or impregnated with a filler material of film forming polymer that can include fluorocarbon polymers. An intermediate transfer belt without reinforcement is not disclosed.
Conventional printing apparatuses in the art have used a single imaging station such as a photoreceptor drum or dielectric charge receiver. In such multi-image systems, each image is formed on the imaging member at an image forming station and then each image is developed at a developing station and transferred to an intermediate transfer member. Each of the images may be formed and developed on the imaging member sequentially and then the multi-image developed on the imaging member finally transferred to the intermediate transfer member, or, in an alternative method, each image may be formed on the imaging member, developed, and transferred to the intermediate transfer member, whereupon the imaging member will be cleaned before receiving the next image which, following development, is transferred in registration onto the prior image on the intermediate transfer member.
The multi-image system could be a color copying system or printing system in which each color of an image being copied is formed on the imaging member. Cyan, yellow, magenta and black are four toner colors commonly used in such color copying systems. As in the multi-image system above, each of the colored images may be formed and developed on the imaging member sequentially and then transferred to the intermediate transfer member, or, in the alternative, each color of an image may be formed on the imaging member, developed, and transferred to the intermediate transfer member in registration.
U.S. Pat. Nos. 4,403,848 and 4,833,503 (both to Snelling) disclose a printing system utilizing a single photoreceptor to produce a multi-color image. In such systems, the photoreceptor imaging member is charged and each color exposed and developed at spaced intervals on the photoreceptor. The photoreceptor is recharged following development of the preceding color but prior to exposure of the succeeding color. Buffers control the timing of the individual color images to assure registration of the color images with one another. Neither Snelling patent discloses the use of an intermediate transfer member, instead transferring the multi-color image directly from the imaging station to the substrate.
The conventional methods described above using a single imaging station generally require that the intermediate transfer member have rigidity in order to maintain registration of the image upon the intermediate transfer member during transfer from the imaging member. Thus, a substrate or other reinforcing member has been used to provide the needed rigidity. Transfer of an image from the imaging member to the intermediate transfer member is usually effected by electrostatic means, so that the substrate should be electrically semiconductive. Transfer of the image from the intermediate transfer member to an image receiving substrate is often accomplished with the aid of heat and pressure, so that the substrate should also be thermally conductive. Examples of conventional substrate materials include polyesters, polyimides, stainless steel and numerous metallic alloys.